Are you sure they're all deleted or you are just not seeing them?
Ctrl-shift-E brings up the extension window.
If they're all there, means that they're just not visible, a side effect on this new snapshot when started the first time if the extension toggle was not ticked in settings.
Alt-P for Settings, Address Bar, tick "Extensions Toggle"
Now they should be visible again, and they can be also moved to have them in the order you want.
You can also untick Extension toggle now, closing and reopening Vivaldi should leave the extensions icon visible.